Control valve means for fluid motors

ABSTRACT

A pressure reducing valve mechanism maintains pressure in one of the service passages of a control valve at a desirably low value except at times when the control valve element is actuated to a position effecting flow of pressurized supply fluid to said service passage for delivery to a motor governed by the control valve.

nited States Patent 1 Tennis CONTROL VALVE MEANS FOR FLUID MOTORS t 1 52 Us. 0.... ..l37/596, 137 5962 [51] Int. Cl ..Fl6k 11/00 [58 Field of Search 137/5962; 596 B,

1 51 May 1, 1973 [56] References Cited UNITED STATES PATENTS 7 3,285,282 11/1966 Martin ..l37/596 4/l969 Stacey ..l37/596.2 x

Primary Examiner-Henry T. Klinksiek Assistant Examiner-Robert J. Miller Attorney-Ira Milton Jones [5 7 1 ABSTRACT A pressure reducing valve mechanism maintains pressure in one of the service passages of a control valve at a desirably low value except at times when the controlvalve elementis actuated-to a position effecting flow of pressurized supply fluid to said service passage for delivery to a motor governed by the'control valve.

14 Claims, 3 Drawing Figures 2 Sheets-Sheet 2 m VE NTOR V P1 521015; H'.Tez1ms ATTORNEY Patented May 1,

BACKGROUND OF THE INVENTION This invention relates tocontrol valves for governing the operation of fluid motors, and it has more particu- .lar reference to valve instrumentalities by which the speed of motor operation can be closely controlled despite load influences which tend to drive the motor at rates faster than desired.

The tendency for fluid motors such as hydraulic cylinders to be driven by the load thereon is well known. The booms of backhoes and front end loaders, for example, constitute heavy loads which are customarily raised and lowered by hydraulic cylinders at the dictate of control valves therefor. During lowering of such a boom, the control valve for its cylinder directs pressure fluid from a pump into one end of the cylinder and conducts fluid expelled from the other end thereof to the reservoir line of the system. The boom, aided by gravity, tends to descend rapidly at an accelerating and uncontrolled rate, and the boom usually attains a speed such that the expanding end of its cylinder cannot'be kept filled with fluid from the pump. When that occurs, a void is drawn in the expanding end of the cylinder, and positive control over the boom is lost until its cylinder is refilled with fluid.

In situations where the load is swingable from side to side, as is the case with the boom of a backhoe, the load can actually drive the cylinder in each direction. This makesit extremely difficult to control the speed with which the boom is swung, and positive control over the boom at all times cannot be had.

Elaborate and costly throttling schemes have been proposed in the past in an effort to achieve controlled movement of heavy loads by their hydraulic cylinders. This invention provides an exceptionally simple and low cost solution to that problem.

SUMMARY OF THE INVENTION It is the primary object of this invention to provide a control valve instrumentality for a fluid motor, wherein exceptionally good control over the speed of motor operation is achieved by means which comprises fluid pressure responsive mechanism to effectively throttle the flow of motor return fluid in one or in both service passages of the valve.

More specifically, it is an object of the invention to so combine with a more or less conventional control valve a fluid pressure repsonsive exhaust throttling device which can comprise a simple pressure reducing valve mechanism for one or for both service passages of the control valve to maintain pressure therein at a substantially uniformly low value above that of fluid in the return passages of the control valve.

It is also a purpose of the invention to provide fluid motor control instrumentalities such as described in the preceding object with means to render the pressure reducing valve mechanism or mechanisms ineffective at times when pressure fluid is being directed to the servalve for a fluid motor as to minimize leakage of fluid past the control spool of the valve to the return embodying this invention;

passages in the neutral position of the control valve element.

With these observations and objectives in mind, the manner in which the invention achieves its purpose will be appreciated from the following description and the accompanying drawings, which exemplify the invention, it being understood that changes may be made in the specific apparatus disclosed herein without departing from the essentials of the invention. setforth in the appended claims.

The accompanying drawings illustrate two complete examples of the embodiments of the invention constructed according to the best modes so far devised for the practical application of the principles thereof, and in which:

FIG. 1 is a sectional view of a hydraulic control valve FIG. 2 is a fragmentary sectional view corresponding to the left hand portion of FIG. 1 but showing an operating position of the control valve spool; and

FIG. 3 is a fragmentary sectional view corresponding to a portion of FIG. 1 but showing a modified embodiment of the invention.

Referring now to the accompanying drawings, the numeral 5 generally designates a hydraulic control valve of more or less conventional construction except as to features to be described hereinafter. The control valve comprises a body 6 having a bore 7 thereinand a valve spool8 slidable axially in the bore. The spool can be shifted in either direction from the neutral or hold position thereof seen in FIG. 1 to a pair of operating positions, one of which is seen in FIG. 2, to connect either of a pair of service passages 9 and 10 of the valve with the pressure fluid inlet thereof (not shown) through what can be termed a high pressure supply or bridge passage 11, and to concurrently connect the other service passage with a reservoir port (not shown) through an exhaust passage adjacent thereto.

In the valve shown, the exhaust passages 12 and 13, open to the bore at widely spaced locations along its length. The bridge passage 11 is U-shaped, and has legs 11a and 11b which open to the bore 7 at zones spaced .axially inwardly from the junctions between the exhaust passages and the bore. Each service passage connects with the bore at a zone between one of the exhaust passages and the leg of the bridge passage adjacent thereto. f

In either operating position of the control valve spool 9, pressure fluid from the valve inlet is diverted into the high pressure bridge passage ll through a load holding check valve 13 in a conventional way. It should be noted, however, that the check valve is held in place in the valve body by means of a threaded plug 14 which extends down into the body and has a well 15 therein opening to the bridge passage. A :stem 16 on the check passages thereof from one or both of its service valve projects up into the well 15 to be guided thereby for opening and closing motion axially of the well. As is customary, the check valve stem has an angled passageway 17 therein to communicate the inner portion of the well 15 with the bridge passage and thus enable fluid to be readily expelled to the latter during opening motion of the check valve. A light spring (not shown) is ordinarily accommodated in the well to yieldingly urge the check valve toward closed position.

Mounted on the body 6 of the control valve directly over the mouths of its service passages 9 and 10, are pressure reducing valve mechanism 20 and 21, respectively. Since the pressure reducing valve mechanisms are identical, a description of the left hand mechanism 20 will suffice for both.

The pressure reducing valve mechanism 20 comprises a body 22 having a bore 23 therethrough, closed at its opposite ends by means of plugs 24 and 25. Those portions of the bore which are axially inwardly adjacent to the plugs 24 and 25 provide chambers 26 and 27, respectively. Slidably mounted in the bore 23 with its opposite end portions received in the chambers 26 and 27 is a fluid pressure sensitive plunger 29 having a circumferential groove 30 therein intermediate its ends to normally provide communication through the bore 23 between two branches 31 and 32 of a passageway that forms a continuation of the associated service passage 9. The branch 31 communicates directly with service passage 9 and with the bore 23 at a location spaced axially a short distance from the junction of the other branch 32 with said bore. Hence, the service passage 9 and the two branches 31 and 32 of the passageway in the pressure reducing valve mechanism constitute continuous portions of a service line through which pressure fluid can flow to and from a fluid motor.

A spring 34 confined between the plug 25 and an adjacent end portion of the plunger 29 urges the latter toward a normal or inactive position defined by the plug 24. In this normal position of the plunger, its circumferential groove 30 spans the junctions between the bore and both passage branches 31 and 32 and thus communicates the branch 32 with the associated service passage 9.

The chamber 26 at the left hand end of the plunger 29 is communicated with the branch passage 31 and hence with the service passage 9 by means of a passageway in the plunger. That passageway has an axial branch 36 which opens to the chamber 26 and a radial branch 37 which opens to the branch passage 31 and at all times communicates with the service passage 9. Accordingly, the chamber 26 will contain fluid at a pressure corresponding to that in the service passage 9.

The chamber 27 at the other end of the plunger 29 is vented so that the plunger can move to the right in response to pressure of fluid in chamber 26. Chamber 27 can be communicated in any desired fashion with the reservoir of a system of which the control valve forms a part, or it can be vented to the bridge passage in the interior of the control valve body 6 as by means ofa vent line 40 shown diagrammatically as connecting with the plugs 14 and 25 in communication with axial ports 41 and 42, respectively, in said plugs. The vent line 40 thus serves to communicate the chamber 27 with the well in the interior of the plug 14, and with the high pressure bridge 11 through the angled passage 17 in check valve 13.

One ,of the features of this invention resides in providing the control spool 8 with a passageway 44 having axially spaced radial branches 45 to communicate one leg 11a of the high pressure bridge passage 11 with the return passage 12 in the neutral or hold position of the control valve spool 8. By such venting of the bridge passage, the chamber 27 of the pressure reducing valve mechanism is also vented in the hold position of the valve spool 8, and the plunger 29 can respond to load pressurized fluid in the chamber 26 and be acuated to the right thereby and thus effect reduction of pressure in the service passage 9 to a value determined by the force of the spring 34 opposing such response of the plunger.

Thus, with a 200 p.s.i. spring, for example, the pressure reducing valve mechanism will function to reduce pressure to about 200 p.s.i. in the service passage 9,

whereas the pressure therein might otherwise reach a value as high as 3,000 p.s.i. if the governed cylinder is heavily loaded. Consequently, with zero pressure in the adjacent exhaust passage 12, the leakage of fluid thereto along the wall of the spool receiving bore 7 from thes ervice passage 9 when such reduced pressure obtains in the lattercould be as much as one-fifteenth less than the leakage that would occur if a non-reduced pressure of 3,000 p.s.i. were allowed to remain in the service passage in the hold position of the valve spool 8. In other words, the leakage would be far greater with a 3,000 p.s.i. pressure differential between the exhaust passage 12 and service passage 9 than it is with only a 200 p.s.i. pressure differential between said passages.

The pressure reducing valve mechanism 20 will maintain this desirably low pressure differential between passages 9 and 12 in the neutral position of the valve spool 8 as long as seepage past the left hand land of plunger 29 to the service passage 9 is no greater than the leakage from service passage 9 to the adjacent exhaust passage 12 across the control valve spool 8. This can normally be taken care of by assuring adequate land length along the wall of the bore 23 between its junctions with passage branches 31 and 32.

It will thus be seen that the pressure reducing valve mechanism will function to minimize leakage of pressure fluid from either service passage to the adjacent exhaust passage, past the control valve spool 8 in the neutral or hold position of the latter.

The control valve spool 8 is shown in an operating position in FIG. 2 which, for the sake of discussion, can be considered as permitting pressure fluid to exhaust to passage 12 from a single acting hydraulic cylinder, via service passage 9. The valve spool has not been moved to a full operation position to the left of neutral, but is in a partial operating position at which it meters flow of exhaust fluid to the return passage 12. This situation represents the heretofore commonly accepted way of governing the speed of motor operation, in this case, the rate at which the load on the cylinder is allowed to descend.

If the load on the governed cylinder is a heavy one that tends to descend at an increasing rate, exhaust fluid will be forced through the metering orifice set up by the control valve spool at correspondingly faster rates. Under such conditions, the valve spool 8 can only effect throttling over a very narrow range, through the metering notches ordinarily provided in the spool lands governing exhaust flow.

This problem is solved by the pressure reducing valve mechanism 20 for service passage 9. Again let it be assumed that the spring 34 exerts a 200 p.s.i. force upon the plunger 29 tending to hold it in its wide open or inactive position allowing free flow of fluid to service passage 9 from branch passage 32. The plunger 29 will then respond to increase in pressure in service passage 9 to a value above 200 psi. and be moved tothe right by such pressure in chamber 26, toward a closed position restricting communication between passage branch 32 and the service passage 9 whatever extent is necessary to restore the desired 200 p.s.i. differential in the pressures of fluid in the service passage 9 and in the adjacent exhaust passage 12. Any tendency for increase in pressure in service passage 9 due to the ten .dency for the load on the governed cylinder to descend at an accelerating rate will thus be manifested in further automatic adjustment of the plunger 29 in the passage closing direction, to keep the pressure in service passage 9 at a constant low value predetermined by the strength of spring 34.

Other advantages result from the use of the pressure reducing valve mechanisms described. For example,

' tion of the speed of descent of a single acting cylinder,

(or of the speed of rotation of a rotary fluid motor supplied with pressure fluid from the other service passage 13), it should not interfere with flow of pump fluid to a cylinder or other motor when the valve spool 8 is actuated to an operating position communicating the ser vice passage 9 with the adjacent leg of the high pressure bridge passage 11, and the load is to be raised. For that purpose, the vent line 40 which connects the spring .8 is in a left hand operating position directing pump fluid from the supply bridge 11 to service passage for flow to one end of a hydraulic cylinder while chamber 27 of the pressure reducing valve mechanism H with the high pressure bridgepassage 11 also serves to render the plunger 29 non-responsive to the highpressure of pump fluid flowing through the service passage 9 to the cylinder. This results from the fact that fluid in spring chamber 27 will then be maintained atthe same high pressure as obtains in the bridge passage 11, and the plunger 29 will be held thereby in its wide open limit of motion defined by its engagement with plug 24.

Pressure reducing valve mechanisms 20 and 21, such as seen in FIG. 1, are useful for both service passages of a control valve governing the operation of reversible fluid motors such as double acting hydraulic cylinders. The operation of the pressure reducing valve 21, of course, is the same as that of the pressure reducing valve mechanism 20 described above. It will function to maintain a desirably low pressure in service passage 13, in the neutral position of valve spool 8 as well as when the spool is in a position communicating service passage 10 with the adjacent branch 13 of the exhaust passage; but its vent line connection with the bridge passage 11 renders it ineffective at timeswhen its associated service passage l0is communicated with the leg 11b of the bridge passage by the control valve spool.

Another important featureof the invention which results from venting the chambers 27 of the two pressure reducing valves 20 and 21 to the high pressure bridge or supply passage 11 of the control valve, is that this expedient permits the pressure reducing valves to achieve a counter balancing or flow coordinating funcproviding for return flow of fluid from the other end of the cylinder to exhaust passage 12 through service passage 9 and the pressure reducing valve 20 therefor. At that time, the pressure of supply fluid is manifested in the springchambers 27 of both pressure reducing valves through the venting lines 40, 41 and 42 and the angled passage 17 in load check 13. Hence, the pressure will immediately rise in the supply bridge 1 I if and when the plunger of valve 20 occupies a restricting position at which it will not allow flow of fluid out of the exhausting endof the governed cylinder at a high enough'rate to match the flow of pump fluid into its other end. Such rise in pressure, of course, is also manifested in the chambers 27 of both pressure reducing valves 20 and 21. As a result, the pressures in the chambers 26 and'27 of valve 21 will be in balance at the increased pressure and the spring 34 thereof will hold the valve plunger in its open position; but the in; creased pressure in the spring chamber 27 of the pressure reducing valve 20 on the return side will cause the plunger thereof to be moved in the valve opening or flow coordinating direction to allow exhaust flow from the governed cylinder as fast as pump fluid is allowed to sociated pressure reducing valve mechanism, at times when the spring chambers 27 thereof are vented to a reservoir or to theopposite service passage 10, instead of to the bridge passage 11. As diagrammatically seen in FIG; 3, a check valve controlled bypass line 50 can then be provided to connect the branch passages 31 and 32, in bypass relation to the plunger 29. The check valve 51 in the bypass line is oriented to permit pressure fluid to flowsubstantially freely from the branch passage 31 to the branch passage: 32, regardless of the position of the plunger 29 of the pressure reducing valve mechanism, while precluding reverse flow through the bypass line. 7

As will be appreciated, the bypass line 50 and its check valve 51. can be incorporated in the body of the pressure reducing valve mechanism if desired. Similarly, the pressure reducing valve mechanisms can be incorporatedin the body 6 of the control valve, or secured thereto by screws, whichever is preferred.

From the foregoing description, together with the accompariying drawings, it will be' readily apparent to those skilled in the art that this invention provides an exceptionally simple but efficacious way of utilizing pressure reducing valve mechanisms to help govern the speed at which fluid motors are operated by their coritrol valves.

The invention is defined by the following claims:

1. Valve instrumentalities for governing operation of a fluid motor, characterized by:

A. a control valve having a pair of service passages for connection with the opposite sides of a fluid motor, and a valve element movable through a number of metering positions to full flow operating positions concurrently communicating either of the service passages with pressure fluid supply means and the other service passage with fluid return means in the valve, from a hold position closing off said service passages from the fluid supply and return means;

B.- a pressure reducing valve mechanism for each service passage, having a passageway through which exhaust fluid from the governed motor flows to the fluid return means through the associated service passage in one of said full flow operating positions of the valve element, and a pressure responsive valve member actuatable in a passageway resticting direction whenever the pressure of exhaust fluid in the associated service passage is caused to exceed that of fluid in the return means by a substantially small amount as a result of the valve element occupying one of said metering position, to thereby maintain the pressure of exhaust fluid in the associated service passage at a substantially uniformly low value above that of fluid in the return means;

C. and means cooperable with each pressure reducing valve mechanism and rendered operative in response to pressure of fluid in said pressure fluid supply means for effecting flow of such supply fluid through the associated service passage to the governed motor substantially without interference from said pressure reducing valve mechanism.

2. The combination of claim 1, wherein said last named means comprises a chamber communicated with the pressure fluid supply passage means, and a plunger which is connected with said valve member and which operates in said chamber to be held by pressure of supply fluid therein in a position blocking movement of said valve member toward a passageway restricting position.

3. In combination with a control valve for a fluid motor, having a valve element movable to a first position to communicate a service passage with supply passage means, and movableto a second position to provide for flow of motor exhaust fluid through the service passage to return passage means, said control valve element being movable through a range of metering positions short of said second position thereof to provide for restricted flow of motor exhaust fluid from the service passage to the return passage means in an amount which increases as the valve element approaches its said second position:

A. a pressure reducing valve mechanism having 1. a passageway connecting with the service passage and through which pressure fluid flows both to and from the governed motor,

2. a valve member actuatable in one direction to restrict flow of motor exhaust fluid through said passageway,

3. and a spring to yieldingly urge the valve mmber toward an inactive position affording substantially free flow of motor exhaust fluid through said passageway;

B. means rendered operative in response to the rise in service passage pressure produced by movement of the control valve element to one of its said exhaust flow metering positions for effecting actuation of the pressure reducing valve member in the flow restricting direction an extent depending upon the magnitude of the pressure present in the service passage;

C. and means rendered operative in response to the pressure of supply fluid flowing to the service passage in said first position of the control valve element, for effecting flow of such supply fluid through said passageway and to the governed motor substantially without interference from the pressure reducing valve member.

4. The combination of claim 3, further characterized A. said control valve element being movable to a load holding third position to close off the service passage from the fluid supply and return passage means;

B. and said pressure reducing valve mechanism also being operable to maintain the pressure in the service passage at said low value in said load holding position of the valve element.

5. The combination of claim 3, wherein said last named means comprises:

A. means connected with the service passage providing a bypass around the pressure reducing valve mechanism, through which pressure fluid can flow to the governed motor in said first position of the valve element;

B. and a check valve in said bypass to block reverse flow of fluid through the bypass.

6. Valve instrumentalities for governing flow of pressure fluid to and from a fluid motor via a service line connectable therewith, characterized by:

A. a directional control valve having fluid supply and return passage means, a service passage for connection with the service line to form a part thereof, and a valve element movable from a hold position to first and second operating positions to respectively communicate the service passage with the fuild supply and return passage means, said valve element being movable through a range of metering positions to provide for return flow of motor exhaust fluid from theservice passage to the return passage means in an amount which increases with movement of the valve element toward said second position thereof and reaches maximum thereat;

B. a pressure reducing valve mechanism having a valve member to regulate flow of motor exhaust fluid through the service line to the service passage, said valve member being urged by a spring toward a normally inactive position but being actuatable out of said position in a direction to restrict return flow of motor exhaust fluid through said service line to the service passage under force exerted on the valve member counter to the force of its spring;

C. means for monitoring the pressure of fluid present in the service passage and rendered operative by the valve element in said metering positions thereof to translate said monitoring pressure into said actuating force on the valve member'whenever said monitored pressure rises to a predetermined value above that of fluid in the return passage means;

D. and means rendered operative by supply fluid pressure, in said first operating position of the valve element, for assuring substantially free flow of pressure fluid through the service line to a fluid motor connected therewith.

7. The valve instrumentalities of claim 6 wherein said last named means comprises:

A. means defining a piston connected with said valve member and which can be held by fluid pressure force in a position blocking movement of the valve member toward its active position;

B. and means for translating pressure of supply fluid into said force on the piston.

8. The valve instrumentalities of claim 6, wherein wherein said last named means comprises:

A. a passage connecting with the service passage and through which supply fluid can flow in bypass relation to said valve member in said first operation position of the control valve element;

B. and a check valve in said bypass passage to block flow of exhaust fluid therethrough to the return means in said second operating position of the control valve element.

9. The valve instrumentalities of claim 6, further characterized by:

A. said pressure reducing valve mechanism comprising a pair of chambers, one of which is communicated with said service passage to be pressurized by motor exhaust fluid therein;

B. said valve member having portions disposed in said chambers, and being movable toward an ac tive position restricting said passageway in response to pressure of motor exhaust fluid in said one chamber;

C. and means venting the other of said chambers.

10. The combination of claim 9, wherein said last named means comprises a vent line at all times connecting said other chamber with pressure fluid supply passage means in the control valve.

1 1. The combination of claim 9., wherein said venting means communicates with the pressure fluid supply passage means in tne control valve.

12. The combination of claim 9., wherein said venting means comprises a duct at all times communicated with the fluid return means.

13. The combination of claim 9, wherein said venting means comprises a duct which communicates with the pressure fluid supply passage means in the control valve and further comprises a passage in the control 

1. Valve instrumentalities for governing operation of a fluid motor, characterized by: A. a control valve having a pair of service passages for connection with the opposite sides of a fluid motor, and a valve element movable through a number of metering positions to full flow operating positions concurrently communicating either of the service passages with pressure fluid supply means and the other service passage with fluid return means in the valve, from a hold position closing off said service passages from the fluid supply and return means; B. a pressure reducing valve mechanism for each service passage, having a passageway through which exhaust fluid from the governed motor flows to the fluid return means through the associated service passage in one of said full flow operating positions of the valve element, and a pressure responsive valve member actuatable in a passageway resticting direction whenever the pressure of exhaust fluid in the associated service passage is caused to exceed that of fluid in the return means by a substantially small amount as a result of the valve element occupying one of said metering position, to thereby maintain the pressure of exhaust fluid in the associated service passage at a substantially uniformly low value above that of fluid in the return means; C. and means cooperable with each pressure reducing valve mechanism and rendered operative in response to pressure of fluid in said pressure fluid supply means for effecting flow of such supply fluid through the associated service passage to the governed motor substantially without interference from said pressure reducing valve mechanism.
 2. The combination of claim 1, wherein said last named means comprises a chamber communicated with the pressure fluid supply passage means, and a plunger which is connected with said valve member and which operates in said chamber to be held by pressure of supply fluid therein in a position blocking movement of said valve member toward a passageway restricting position.
 2. a valve member actuatable in one direction to restrict flow of motor exhaust fluid through said passageway,
 3. and a spring to yieldingly urge the valve mmber toward an inactive position affording substantially free flow of motor exhaust fluid through said passageway; B. means rendered operative in response to the rise in service passage pressure produced by movement of the control valve element to one of its said exhaust flow metering positions for effecting actuation of the pressure reducing valve member in the flow restricting direction an extent depending upon the magnitude of the pressure present in the service passage; C. and means rendered operative in response to the pressure of supply fluid flowing to the service passage in said first position of the control valve element, for effecting flow of such supply fluid through said passageway and to the governed motor substantially without interference from the pressure reducing valve member.
 3. In combination with a control valve for a fluid motor, having a valve element movable to a first position to communicate a service passage with supply passage means, and movable to a second position to provide for flow of motor exhaust fluid through the service passage to return passage means, said control valve element being movable through a range of metering positions short of said second position thereof to provide for restricted flow of motor exhaust fluid from the service passage to the return passage means in an amount which increases as the valve element approaches its said second position: A. a pressure reducing valve mechanism having
 4. The combination of claim 3, further characterized by: A. said control valve element being movable to a load holding third position to close off the service passage from the fluid supply and return passage means; B. and said pressure reducing valve mechanism also being operable to maintain the pressure in the service passage at said low value in said load holding position of the valve element.
 5. The combination of claim 3, wherein said last named means comprises: A. means connected with the service passage providing a bypass around the pressure reducing valve mechanism, through which pressure fluid can flow to the governed motor in said first position of the valve element; B. and a check valve in said bypass to block reverse flow of fluid through the bypass.
 6. Valve instrumentalities for governing flow of pressure fluid to and from a fluid motor via a service line connectable therewith, characterized by: A. a directional control valve having fluid supply and return passage means, a service passage for connection with the service line to form a part thereof, and a valve element movable from a hold position to first and second operating positions to respectively communicate the service passage with the fuild supply and return passage means, said valve element being movable through a range of metering positions to provide for return flow of motor exhaust fluid from the service passage to the return passage means in an amount which increases with movement of the valve element toward said second position thereof and reaches maximum thereat; B. a pressure reducing valve mechanism having a valve member to regulate flow of motor exhaust fluid through the service line to the service passage, said valve member being urged by a spring toward a normally inactive position but being actuatable out of said position in a direction to restrict return flow of motor exhaust fluid through said service line to the service passage under force exerted on the valve member counter to the force of its spring; C. means for monitoring the pressure of fluid present in the service passage and rendered operative by the valve element in said metering positions thereof to translate said monitoring pressure into said actuating force on the valve member whenever said monitored pressure rises to a predetermined value above that of fluid in the return passage means; D. and means rendered operative by supply fluid pressure, in said first operating position of the valve element, for assuring substantially free flow of pressure fluid through the service line to a fluid motor connected therewith.
 7. The valve instrumentalities of claim 6 wherein said last named means comprises: A. means defining a piston connected with said valve member and which can be held by fluid pressure force in a position blocking movement of the valve member toward its active position; B. and means for translating pressure of supply fluid into said force on the piston.
 8. The valve instrumentalities of claim 6, wherein wherein said last named means comprises: A. a passage connecting with the service passage and through which supply fluid can flow in bypass relation to said valve member in said first operation position of the control valve element; B. and a check valve in said bypass passage to block flow of exhaust fluid therethrough to the return means in said second operating position of the control valve element.
 9. The valVe instrumentalities of claim 6, further characterized by: A. said pressure reducing valve mechanism comprising a pair of chambers, one of which is communicated with said service passage to be pressurized by motor exhaust fluid therein; B. said valve member having portions disposed in said chambers, and being movable toward an active position restricting said passageway in response to pressure of motor exhaust fluid in said one chamber; C. and means venting the other of said chambers.
 10. The combination of claim 9, wherein said last named means comprises a vent line at all times connecting said other chamber with pressure fluid supply passage means in the control valve.
 11. The combination of claim 9, wherein said venting means communicates with the pressure fluid supply passage means in tne control valve.
 12. The combination of claim 9, wherein said venting means comprises a duct at all times communicated with the fluid return means.
 13. The combination of claim 9, wherein said venting means comprises a duct which communicates with the pressure fluid supply passage means in the control valve and further comprises a passage in the control valve element effective only in the hold position thereof to communicate the pressure fluid supply passage with the fluid return means.
 14. The combination of claim 9, wherein said venting means comprises a duct communicating said other chamber with the pressure fluid supply passage means, and wherein passage means in the control valve element communicates the pressure fluid supply passage means with the return means in the hold position of the valve element. 